One of the major passenger complaints with aircraft travel is noise within the cabin section. Noise develops in an aircraft cabin from several sources. The most common sources are internally or externally mounted moving components, such as a transmission, engine or rotor system. Another source of cabin noise is air flows over various aircraft fuselage components, such as engines, landing gear, and cowlings. These components generate vibrations in the aircraft that propagate through the airframe and radiate into the cabin.
Noise is a particular problem in helicopter cabins since the rotor and transmission systems produce a significant amount of vibration in the airframe structure. This problem is more pronounces in helicopters than in fixed wing aircraft inasmuch as the dynamic components on a helicopter are mounted directly above the cabin, whereas the dynamic components on a fixed wing aircraft are mounted on the wings. These vibrations travel through the airframe structure and into the cabin interior resulting in structurally radiated noise due to vibration. It is not uncommon for noise levels within an uninsulated aircraft to reach upwards of 110 decibels or more, which can be very annoying and even harmful to the occupants.
The main noise problem in helicopter cabins is mid to high frequency gear whine noise from the main transmission. The main transmission of most helicopters is transmitting from several hundred to several thousand horsepower and is bolted directly to the cabin ceiling structure. This results in cabin noise vibrations typically from about 350 Hz through 4,000 Hz. In contrast noise vibrations from the main and tail rotor sources are in the 20 Hz to 125 Hz range and are attenuated by up to 40+ dB by the response of the human ear.
Aircraft cabin interiors are generally designed to keep aircraft interior noise below a certain level predetermined by competitive pressures in the marketplace. For example, the civilian S-76 aircraft made by Sikorsky Aircraft Corporation for executive transport has a design average noise level limit with the environmental control system (fans, vent air and cooling/heating system) turned off of approximately 75 dB SIL4. The SIL4 (Speech Interference Level 4) noise measurement metric is the arithmetic average of the sound pressure levels in the 500, 1000, 2000 and 4000 Hz octave bands. It rates steady noise according to it's ability to interfere with conversation between two people. The SIL4 noise measurement metric was developed by an engineer in the U.S. Navy in the 1950's and has been adopted as an ANSI (American National Standards Institute) standard.
Various attempts have been made by the assignee of the present invention to reduce noise levels within the cabin to below these values. One attempt was to incorporate a constrained damping layer on the airframe structure, i.e., frames, beams and skins. The constrained damping layer consisted of a layer of elastomer bonded to the airframe structure with an aluminum sheet bonded to the top. The constrained damping layer was designed to shift the location of the maximum shear loads from the airframe to the elastomer damping layer. The vibration induced shearing of the constrained elastomer damping layer assisted in dissipating the vibratory energy in the structure, in turn, reducing structurally radiated noise. The primary drawback to this design was that the damping elements had to be mounted at 102 separate locations which was very time consuming. A typical aircraft had to sit on the assembly line for an additional 1-2 days to install the constrained damping layers and allow the adhesive to cure.
A second attempt to reduce noise involved mounting 22 tuned high frequency (778 Hz., the primary gear mesh tone in the Sikorsky S-76 helicopter cabin) vibration absorbers around the cabin section of the aircraft. These absorbers were mounted directly on the airframe structure at locations defined during testing to have high vibratory levels on the cabin interior side of the airframe structure. While the vibration absorbers were effective, they had to be specifically tuned to the particular aircraft structure since each aircraft experiences slightly different vibrational loads. This again was a very tedious and time consuming effort substantially impacting the aircraft production process.
Other attempts were made to reduce the noise levels, such as the incorporation of tuned absorbers (again tuned to 778 Hz) on the aft end of the main transmission support beams above the cabin and the addition of weights to produce a vibration impedance mismatch on and/or near the foot of the transmission. Both of these attempts proved to be very heavy and very sensitive to small frequency variations.
A more recent attempt has been to use noise reduction or soundproofing blankets. As shown in FIGS. 1 and 1A, the blankets B are adhesively attached directly to the airframe skins S and frames/beams F in the upper bays of the aircraft cabin section where much of the noise is initially radiated into the cabin interior. The blankets B consist of a sandwich structure including a lower foam layer LF approximately 1/4 inch thick, an approximately 1/8 inch thick vinyl mass barrier layer V, and an upper foam layer UF approximately 1/4 to 1 inch thick. A vapor barrier VB made from a polyvinyl fluoride material sold by E. I. du Pont De Nemours and Company under the trade name TEDLAR surrounds the sandwich structure to prevent handling damage to the foam layers and prevent fluid absorption (water, oil, hydraulic fluid, etc.) by the foam layers.
This blanket arrangement proved to work very well at reducing noise. The blankets were also moderately easy to install since they included an adhesive film layer AF on the side of the blanket that was to be attached to the cabin interior trim panels P. However, the main deficiencies with this prior blanket arrangement were that it was very heavy (reducing the aircrafts' range and payload) and the blanket was relatively stiff making the blankets very difficult to physically install in the cabin overhead structure area.
A need, therefore, exists for an improved sound absorbing blanket for reducing noise in an aircraft cabin which is lightweight and easy to install.